High-speed electro-optical light gates have been known heretofore which comprise twisted nematic liquid crystal devices positioned within a light polarizing system to reduce the turn-on and turn-off optical transmission response times of the light gate. Such a light gate is disclosed by Tobias, International Handbook of Liquid Crystal Displays, OVUM Ltd., London, England, 1975, paragraph 9.5.3, FIGS. F9.19 and F9.20. This prior art electro-optical light gate is shown in FIG. 1.
With reference to FIG. 1, the light gate 10 includes a fast shutter 12 positioned in front of a slow auxiliary shutter 14. Fast shutter 12 includes a pair of twisted nematic liquid crystal devices 16 and 18 which are disposed face-to-face and are positioned between a pair of neutral linear polarizing filters 20 and 22. Polarizing filter 20 has vertical absorption axis 24 and horizontal transmission axis 26, and polarizing filter 22 has vertical transmission axis 28 and horizontal absorption axis 30.
Light rays emitted from an external light source (not shown) are received by polarizing filter 20 and exit through polarizing filter 22 of fast shutter 12. Each one of the liquid crystal devices 16 and 18 is optically active and is characterized as having "ON" and "OFF" switching states. In the "ON" state, the presence of a voltage signal of the proper magnitude across the device leaves unchanged the direction of polarization of light rays passing therethrough. In the "OFF" state, the absence of a voltage signal across the device imparts a 90.degree. rotation to the direction of polarization of light rays passing therethrough. The twisted nematic liquid crystal device has a shorter response time when switching from the "OFF" state to the "ON" state (i.e., turn-on time) than that when switching from the "ON" state to the "OFF" state (i.e., turn-off time).
With reference to FIG. 2, both liquid crystal devices 16 and 18 are in the "OFF" state at a time prior to T.sub.1 (lines A and B), during which time the light rays passing through horizontal transmission axis 26 of polarizing filter 20 undergo no net change in polarization direction and are completely absorbed by horizontal absorption axis 30 of polarizing filter 22. Prior to time T.sub.1, therefore, no light rays exit fast shutter 12 (line C).
At time T.sub.1, a 120 volt AC signal is applied to either one of liquid crystal devices 16 and 18 (line A) to rotate by 90.degree. the direction of polarization of light rays passing through horizontal transmission axis 26 of polarizing filter 20 and cause the light rays to exit fast shutter 12 through vertical transmission axis 28 of polarizing filter 22 (line C). The relatively high voltage is utilized to decrease the turn-on time of the liquid crystal device. This is referred to as the transmitting optical transmission state of fast shutter 12.
At time T.sub.2, a 120 volt AC signal is applied to the other one of the two liquid crystal devices 16 and 18 (line B) which was not activated at time T.sub.1 to provide a net rotation of 0.degree. to the direction of polarization of light rays passing through horizontal transmission axis 26 of polarizing filter 20 and cause the light rays to be completely absorbed by horizontal absorption axis 30 of polarizing filter 22 (line C). This is referred to as the opaque optical transmission state of fast shutter 12.
The transmission and extinction of light rays through fast shutter 12 is accomplished by activating in time sequence both of liquid crystal devices 16 and 18 from the "OFF" states to the "ON" states. The response times for the transitions from the transmitting to the opaque and from the opaque to the transmitting optical transmission states of fast shutter 12 are both characterized by the shorter turn-on time of the liquid crystal device.
The slow auxiliary shutter 14 is positioned at the output of fast shutter 12 to prevent transmission of light through the light gate 10 while liquid crystal devices 16 and 18 relax to the "OFF" states after transmission of light through fast shutter 12 has been terminated. Slow auxiliary shutter 14 includes twisted nematic liquid crystal device 32 which is disposed between polarizing filter 22 and polarizing filter 34. Polarizing filter 22 is an element common to both fast shutter 12 and slow auxiliary shutter 14, and polarizing filter 34 has vertical absorption axis 36 and horizontal transmission axis 38.
The absence of a voltage signal across liquid crystal device 32 imparts a 90.degree. rotation to the direction of polarization of light rays passing through vertical transmission axis 28 of polarizing filter 22 and causes the transmission of the light rays through horizontal transmission axis 38 of polarizing filter 34. This condition exists during the time interval between T.sub.1 and T.sub.2 and for short time periods prior to time T.sub.1 and subsequent to time T.sub.2 (line D) to permit transmission of light through light gate 10 (line C).
After the cessation of light transmission through fast shutter 12 (line C) but prior to the removal of the 120 volt AC signals from liquid crystal devices 16 and 18, a 10 volt AC signal is applied to liquid crystal device 32 (line D) to prevent sporadic transmission of light through light gate 10 due to the mismatched turn-off time response characteristics of the liquid crystal devices 16 and 18 after the voltage signal is removed from them at time T.sub.3 (lines A and B). The 10 volt AC signal is applied to liquid crystal device 32 prior to time T.sub.3 to allow sufficient time for liquid crystal device 32 to reach the "ON" state.
Light gate 10 is capable of high-speed changes in optical transmission for light of all wavelengths. This is accomplished, however, with the use of a large number of optical components which attenuate the intensity of the light rays as they propagate through the light gate. Moreover, the twisted nematic liquid crystal devices incorporated in the fast shutter present a capacitive load which must be overcome by very high voltage AC signals to rapidly switch the devices between the two switching states.